M21 1958

[maddogmargetts@yahoo.co.u]

The old M21 engine is running beautifully but a new noise on the bike has occured which is causing me concern. Trouble is, it is difficult to hear where it is actually coming from. When decelerating I notice a grating rattling kind of noise like a metal part vibrating against another metal part, then it stops as I slow down. Seems to happen around 25 to 30mph when reducing speed and not on accelerating. Could this be something like a slackened chain which is too loose? I know that all parts have their own particular resonance which make them vibrate at certain frequencies. Would a drive chain rattle against another part when decelerating? It has about an inch of movement when pressed. Could there be any other parts which can cause this symptom? What really worries me is that it might be problems inside the gearbox which is far beyond my technical capacity to remedy.

[idie]

It could be the chain an inch of movement is rather excessive. I would look for half an inch.

[maddogmargetts@yahoo.co.u]

thanks, i will readjust it to half an inch to see if that works. Are the M21 gearboxes known to be good in the sense of being rugged and long lasting or do you know of any weaknesses with them?

[JulianS]

Quite robust boxes. Most M21s spent years with large sidecars attached and many spent years and high miles working for the AA with sidecar attached.

[maddogmargetts@yahoo.co.u]

Yes, that was it. Drive chain needed to be 1/2 inch. Also tightened up the front mudguard and the nuts holding on the petrol tank. One either side of the steering column and one either side of the bit where the tank nears the seat. All the noises were coming from vibration of these parts. I suppose the lesson again is, look at the obvious before going into the more complicated! No need to suspect a faulty gearbox when all the problem is loose bolts !!

[DANFINN]

Always good to check for loose nuts and bolts after a run. I try to do this most times, lost a few nuts and bolts over the years